Avis said it plans to use its fleet of rental cars to beef up Zipcar for its weekend demand, "which is currently constrained by fleet availability."

As a "car sharing" company, Zipcar operates under a slightly different model, appealing to customers who would rather rent a car by the hour, instead of by the day, though it offers services for both. Most rental car companies, including Avis, offer rentals on a daily or weekly basis.